As a light source of a vehicle lamp, substitutions of a semiconductor device such as a light emitting diode (LED) or a laser diode (LD) for a halogen lamp or a high-intensity discharge lamp in the related art have progressed. A drive circuit that drives such a semiconductor light source (hereinafter, referred to simply as a light source) includes a converter that boosts and drops an input voltage such as a battery voltage, and supplies the voltage to the light source and a controller that controls the converter. The controller detects an output current of the converter, and performs a feedback control of a switching of the converter such that the output current approaches a current value corresponding to target luminance.
FIG. 1 is a diagram illustrating a configuration of a vehicle lamp in the related art. FIGS. 2A and 2B are operation waveform diagrams of a vehicle lamp 1r. Referring to FIG. 1, the vehicle lamp 1r includes a light source 10 and a driving device 20r. A voltage VBAT of a battery 2 is input to the vehicle lamp 1r via a switch 4.
The driving device 20r includes a converter 30 and a controller 40r. The converter 30 is, for example, a step-down converter, and drops an input voltage VIN and supplies the voltage to the light source 10 which is a load. The converter 30 mainly includes an input capacitor C11, a switching transistor M11, a rectifier diode D11, an inductor L11, and an output capacitor C12.
The controller 40r detects an output current IOUT flowing in the light source 10 and controls a duty ratio of the switching of the switching transistor M11 such that the output current IOUT approaches a target amount corresponding to the luminance. A current sense resistor R11 is provided on a path of a coil current IL11 in order to detect the coil current IL11 corresponding to the output current IOUT. In the current sense resistor (hereinafter, referred to simply as sense resistor) R11, a voltage drop (hereinafter, referred to as simply sense voltage) VR11 which is proportional to a coil current IL11 occurs.
The coil current IL11 is a pulsating current according to the switching of the switching transistor M11, and the output current IOUT, is the current obtained by smoothing the coil current IL11. The controller 40r stabilizes the coil current IL11 within a target range by a so-called hysteresis control. The controller 40r turns OFF the switching transistor M11 when the sense voltage VR11 reaches the upper threshold value IREFH of the target range, and turns ON the switching transistor M11 when the sense voltage VR11 drops to the lower threshold value IREFL of the target range. An aspect of the coil current IL11 is stabilized within the target range is illustrated in FIG. 2A.